Patent Literature 1 discloses, for example, an IGBT which is a vertical-type power device. An IGBT can be recognized as a combined product of a MOS field effect transistor (MOS-FET) and a bipolar transistor (BJT) and is widely used as a large-current and high-voltage power device in the fields of the industry and household appliances.
As Patent Literature 1 describes, IGBTs can be roughly categorized into a punch-through (PT) type IGBT, a non-punch-through (NPT) type IGBT, and a field-stop (FS) type IGBT which is intermediate between a punch-through type and a non-punch-through type. A PT-type IGBT includes a P conductivity-type (P+) collector layer made of a thick substrate, an N conductivity-type (N−) drift layer formed by epitaxial growth, and an N conductivity-type (N+) buffer layer between the collector layer and the drift layer. A PT-type IGBT is a high-cost device since it has a depletion layer (electric field) in contact with the collector side in an OFF state (reverse bias) and uses an epitaxial wafer. An NPT-type IGBT, on the other hand, includes an N type conductivity (N−) drift layer made of a thinner substrate (silicon wafer) and a P conductivity type (P+) collector layer in the back surface thereof. An NPT-type IGBT is lower in cost, higher in reliability, and only suffers a smaller number of crystal defects because it has a depletion layer not in contact with a collector layer in an OFF state and uses a floating zone wafer, the depletion layer extending from a PN junction of a P-conductivity-type layer and an N-conductivity-type drift layer in the front surface side. A FS-type IGBT includes a drift layer and a collector layer similar to those of an NPT-type IGBT, and an N-conductivity-type buffer layer called a field-stop layer (hereinafter abbreviated to a FS layer) between the drift layer and the collector layer, the N-conductivity-type (N−) substrate being made thinner than the substrate of an NPT-type IGBT. In the FS-type IGBT, a depletion layer (electric field) extending from a PN junction in the front surface is in contact with a FS layer in an OFF state, and the FS layer functions as a stopper of the depletion layer. The FS-type IGBT, which can be thinner than the other two types, is becoming the mainstream of the IGBTs since the recent situation requires the IGBTs, vertical-type power devices to be thinner and thinner to attempt reduction of loss.
A FS-type IGBT can be manufactured in such manners that an N-conductivity-type dopant of phosphorus (P) or antimony (Sb) is implanted into the back surface of a thin N−-conductivity-type substrate to form a FS layer. The FS layer obtained is annealed at high temperatures of 600° C. or higher to be activated, and a shallow P+-conductivity type collector layer is formed in the bottom of the N−-conductivity-type substrate. This method, however, has a following disadvantage. In order to reduce damage of a wafer, the structure for the front surface including a metallizing and a passivation layer is formed in the front surface and a FS layer is thereafter provided in the back surface. The presence of the metal layer in the front surface limits the temperatures of annealing after implantation of the dopant into the back surface to be lower than the temperatures (350° C. to 425° C.) of depositing the passivation layer on the surface. As a result, only a part of the dopant of phosphorus (P) or antimony (Sb) has been annealed and the degree of annealing is significantly varied in a small temperature range. In order to solve the difficulty, Patent Literature 1 describes a technique of forming a FS layer by implanting hydrogen (H) ions, and thereafter generating a shallow P+-collector layer by implanting boron (B) ions, and lastly performing anneal on a wafer for 30 to 60 minutes at temperatures of 300° C. to 400° C. The annealing at the temperatures of 300° C. to 400° C. removes damages caused by the ion implantations and allows the hydrogen in the FS layer to act as a N+-dopant. As has been described, when a FS layer is formed by implanting hydrogen ions, annealing at temperatures of 300° C. to 400° C. can activate the implanted hydrogen and the activated hydrogen can act as a FS layer without damaging the structure for the front surface side (metal and passivation).